本案例的目的是理解如何用Metal实现像素颜色转换滤镜，通过对像素颜色的不同读取方式获取到相应像素颜色，灰度图移除场景中除了黑白灰以外所有的颜色，让整个图像灰度化；

Demo

  • HarbethDemo地址

实操代码

// 转成灰度图滤镜
let filter = C7ColorConvert(with: .gray)// 方案1:
let dest = BoxxIO.init(element: originImage, filter: filter)
ImageView.image = try? dest.output()dest.filters.forEach {NSLog("%@", "\($0.parameterDescription)")
}// 方案2:
ImageView.image = try? originImage.make(filter: filter)// 方案3:
ImageView.image = originImage ->> filter

实现原理

  • 过滤器

这款滤镜采用并行计算编码器设计.compute(kernel: type.rawValue)

/// 颜色通道`RGB`位置转换
public struct C7ColorConvert: C7FilterProtocol {public enum ColorType: String, CaseIterable {case invert = "C7ColorInvert"case gray = "C7Color2Gray"case bgra = "C7Color2BGRA"case brga = "C7Color2BRGA"case gbra = "C7Color2GBRA"case grba = "C7Color2GRBA"case rbga = "C7Color2RBGA"}private let type: ColorTypepublic var modifier: Modifier {return .compute(kernel: type.rawValue)}public init(with type: ColorType) {self.type = type}
}
  • 着色器

取出像素rgb值，然后根据对应像素颜色；灰度图则是取所有的颜色分量，将它们加权或平均；

// 颜色反转，1 - rgb
kernel void C7ColorInvert(texture2d<half, access::write> outputTexture [[texture(0)]],texture2d<half, access::read> inputTexture [[texture(1)]],uint2 grid [[thread_position_in_grid]]) {const half4 inColor = inputTexture.read(grid);const half4 outColor(1.0h - inColor.rgb, inColor.a);outputTexture.write(outColor, grid);
}// 转灰度图
kernel void C7Color2Gray(texture2d<half, access::write> outputTexture [[texture(0)]],texture2d<half, access::read> inputTexture [[texture(1)]],uint2 grid [[thread_position_in_grid]]) {const half4 inColor = inputTexture.read(grid);const half3 kRec709Luma = half3(0.2126, 0.7152, 0.0722);const half gray = dot(inColor.rgb, kRec709Luma);const half4 outColor = half4(half3(gray), 1.0h);outputTexture.write(outColor, grid);
}kernel void C7Color2BGRA(texture2d<half, access::write> outputTexture [[texture(0)]],texture2d<half, access::read> inputTexture [[texture(1)]],uint2 grid [[thread_position_in_grid]]) {const half4 inColor = inputTexture.read(grid);const half4 outColor(inColor.bgr, inColor.a);outputTexture.write(outColor, grid);
}kernel void C7Color2BRGA(texture2d<half, access::write> outputTexture [[texture(0)]],texture2d<half, access::read> inputTexture [[texture(1)]],uint2 grid [[thread_position_in_grid]]) {const half4 inColor = inputTexture.read(grid);const half4 outColor(inColor.brg, inColor.a);outputTexture.write(outColor, grid);
}kernel void C7Color2GBRA(texture2d<half, access::write> outputTexture [[texture(0)]],texture2d<half, access::read> inputTexture [[texture(1)]],uint2 grid [[thread_position_in_grid]]) {const half4 inColor = inputTexture.read(grid);const half4 outColor(inColor.gbr, inColor.a);outputTexture.write(outColor, grid);
}kernel void C7Color2GRBA(texture2d<half, access::write> outputTexture [[texture(0)]],texture2d<half, access::read> inputTexture [[texture(1)]],uint2 grid [[thread_position_in_grid]]) {const half4 inColor = inputTexture.read(grid);const half4 outColor(inColor.grb, inColor.a);outputTexture.write(outColor, grid);
}kernel void C7Color2RBGA(texture2d<half, access::write> outputTexture [[texture(0)]],texture2d<half, access::read> inputTexture [[texture(1)]],uint2 grid [[thread_position_in_grid]]) {const half4 inColor = inputTexture.read(grid);const half4 outColor(inColor.rbg, inColor.a);outputTexture.write(outColor, grid);
}
  • 权值法
const half3 kRec709Luma = half3(0.2126, 0.7152, 0.0722);
const half gray = dot(inColor.rgb, kRec709Luma);
const half4 outColor = half4(half3(gray), 1.0h);
  • 平均值法
const float color = (inColor.r + inColor.g + inColor.b) / 3.0;
const half4 outColor = half4(color, color, color, 1.0h);

总结：
一般由于人眼对不同颜色的敏感度不一样，所以三种颜色值的权重不一样，一般来说绿色最高，红色其次，蓝色最低，最合理的取值分别为Wr ＝ 30％，Wg ＝ 59％，Wb ＝ 11％，所以权值法相对效果更好一点。

对照图

invert bgra brga
gbra grba rbga

Harbeth功能清单

最后